The superior mechanical and strength-to-weight properties of carbon fibers has led to an important class of high performance fiber/matrix composites. These high performance composites are particularly useful for the production of aircraft and automobile body parts for which both strength and light weight are critical. Such composites enable manufacturers to produce relatively light weight structures without sacrificing strength. Consequently, much research has been directed to producing carbon fiber materials with ever increasing high performance properties and physical features that make them more valuable in commercial products and processes.
Various processes have been developed over the years for the production of high performance carbon fiber materials. One of the leading processes for producing high performance carbon fibers is the so-called PAN process wherein polyacrylonitrile is used as a precursor fiber. The PAN process typically starts with a highly prestretched PAN fiber and involves three steps. First is a stabilization treatment wherein the PAN fiber is heat treated in air at a temperature from about 200.degree. to 300.degree. C. for one or more hours. In the second step, the fiber is carbonized at a temperature above about 1100.degree. C. in a non-oxidizing atmosphere. Last is a post heat treatment at temperatures up to about 2500.degree. C. to graphitize the fiber and give it high performance properties. It is in this post heat treatment step that the chemical composition, the crystalline structure, and the mechanical properties are strongly influenced.
There has been an intense effort to develop methods of spinning and carbonizing hydrocarbon pitch fiber to reduce precursor filament cost and weight loss. However, such processes require pitch pretreatment, spinning conditions, and post-treatments to insure correct orientation of carbon atoms in the final products. As a result, use of spun and carbonized hydrocarbon pitch has been nearly as expensive as using the previously noted methods involving organic polymers. Both methods require use of continuous filaments to achieve high orientation and good properties. There is a practical fiber diameter lower limit of 6 to 8 micrometers. Thinner fibers break during spinning and require excessive post-treatment.
An entirely different approach for carbon fiber formation involves the preparation of carbon filaments through the catalytic decomposition at metal surfaces of a variety of carbon containing gases, e.g., CO/H.sub.2, hydrocarbons, and acetone. These filaments are found in a wide variety of morphologies (e.g., straight, twisted, helical, branched) and diameters (e.g., ranging from tens of angstroms to tens of microns). Usually, a mixture of filament morphologies is obtained, frequently admixed with other, non-filamentous carbon (cf. Baker and Harris, Chemistry and Physics of Carbon, Vol. 14, 1978). Frequently, the originally-formed carbon filaments are coated with poorly organized thermal carbon.
The vapor decomposition technique for forming carbon filaments has been extensively studied. U.S. Pat. No. 4,855,091 to Geus prepares carbon filaments by exposing a thermostable substrate covered with reduced metal particles to a carbon-containing gas at 200.degree. to 700.degree. C. U.S. Pat. No. 5,149,584 to Baker et al. deposits a catalyst comprising a group IB element and a second metal which is either iron, nickel, cobalt, or zinc on a carbon fiber substrate and contacts it with carbonaceous material at 500.degree. to 700.degree. C. U.S. Pat. No. 4,565,684 to Tibbetts et al. discloses growing graphite fibers on a suitably nucleated ceramic surface by passing methane gas over the substrate at elevated temperatures, and thickening the initially-formed microscopic carbon filaments by increasing the concentration of methane. Endo et al., "Structural Improvement Of Carbon Fibers Prepared From Benzene," Japanese Journal of Applied Physics, Vol. 15, No. 11, pp. 2073-76 (November, 1976) discloses the preparation of carbon fibers by thermal decomposition of benzene at 1050.degree. to 1080.degree. C. Kato et al., "Formation Of Vapor-Grown Carbon Fibers On A Substrate," Carbon, Vol. 31, No. 7, pp. 989-94 (1992) relates to growing fibers on activated carbon pellets by impregnating the pellets with an iron catalyst and introducing hydrogen sulfide and benzene in a gaseous state with "[n]o carbon fibers . . . produced without the feed of sulfur". U.S. Pat. Nos. 4,663,230, 5,165,909, and 5,171,560 to Tennent disclose the formation of substantially cylindrical carbon fibrils with an outer region of multiple layers of ordered carbon atoms and a distinct inner core region by contacting a metal particle (preferably supported on a refractory material) with a gaseous carbon-containing compound. M. Egashira et al., "Whiskerization of Carbon Beads by Vapor Phase Growth of Carbon Fibers to Obtain Sea Urchin-Type Particles," Carbon, vol. 21, no. 1., pp. 89-92 (1983) produces carbon filaments on hard, non-porous, sulfur-containing carbon beads.
In many cases, these vapor decomposition processes form filaments which can be utilized in high strength applications. There is also some mention that such filaments can be utilized as an electrical conductor or in electrochemical applications, such as for electrodes. See U.S. Pat. No. 4,855,091 to Geus et al. and U.S. Pat. Nos. 4,663,230, 5,165,909, and 5,171,560 to Tennent et al. Unfortunately, the art has been unable to produce carbon filament materials which are commercially useful in such electrical applications. This is due to the low electron transfer rates for such materials. The need, therefore, remains for vapor decomposition products which can be effectively utilized in electrical and electrochemical applications.